更新后：

df = pd.DataFrame({'cast': [['a, b, c'], ['d, e']]})
df['col'] = df['cast'].str[0].str.replace('\s*,\s*', ' ', regex=True)
print(df)

# Output
        cast    col
0  [a, b, c]  a b c
1     [d, e]    d e

旧答案

您可以：

df = pd.DataFrame({'cast': [['a', 'b', 'c'], ['d', 'e']]})
df['col'] = df['cast'].map(' '.join)
print(df)

# Output
        cast    col
0  [a, b, c]  a b c
1     [d, e]    d e